The present invention relates to an electromagnetic variable delay line, and particularly to an electromagnetic variable delay line the delay time of which can be variably controlled according to the operation of a control system. Even more particularly, the present invention relates to such an electromagnetic variable delay line, which particularly is suitable for use as a programmable delay line the delay time of which is variably controlled in relatively small steps according to the value of a digital control signal.
In the prior art, there have been proposed various types of electromagnetic variable delay line. Particularly, in FIG. 8 of the appended drawings, which relates to the prior art, there is shown a circuit diagram of an exemplary such conventional type electromagnetic variable delay line. In this prior art delay line, which is denoted as DLa and is of a distributed constant type, there are comprised a ground electrode which is formed around a cylindrical bobbin, a single layer of solenoid wire, and a plurality of taps denoted as T1 through T7; these arrangements are only schematically shown in the figure. Each of this plurality of eight taps T1 through T7 is connected to one of a corresponding plurality of eight input terminals A0 through A7 of a multiplexer MUX, and this multiplexer MUX further has a plurality of select terminals denoted as D0 through D2. Further, there are provided, as shown in the FIG. 8 circuit diagram, a terminal resistor R and an inverter I.
According to the operation of this electromagnetic variable delay line, a digital signal in binary form for controlling the delay time is supplied to the three select terminals D0 through D2 of the multiplexer MUX, and according to the value of said binary digital signal one of the eight input terminals A0 through A7 of said multiplexer MUX is thereby selected, and thereby the input signal supplied to the input terminal 1 of this electromagnetic variable delay line is supplied, via one of the eight taps T1 through T7 and via the corresponding thus selected one of said eight input terminals A0 through A7, to the output terminal 3 of this electromagnetic variable delay line, with a delay time being imparted to said input signal as characteristic of said selected one of the eight taps T1 through T7. Thus, this device is suitable for use as a programmable delay line as it is.
However, this first type of prior art construction for an electromagnetic variable delay line is subject to a number of problems.
First, if it is desired to make the steps of variation of delay time provided more fine, then the number of the taps of the delay line is required to be increased. However, as the number of bits in the binary control signal increases, the number of taps required increases very rapidly, and from a manufacturing view point the provision of such a large number of taps becomes more and more difficult. For example, if the binary control signal is required to be a four bit control signal, sixteen taps are required; and, if said binary control signal is required to be a six bit control signal, then sixty four taps come to be required; and, even worse, if said binary control signal is required to be an eight bit control signal, then a total of 256 taps come to be required. The provision of such a large number of taps is difficult and costly.
Further, even if it were practicable to provide such a large number of taps, the accuracy of delay time variation provided thereby is deteriorated, and in practice it becomes impractical to vary the delay time in this manner. In other words, because the differences in the input capacitances of the input terminals of the multiplexer MUX, the steps of delay time provided between the taps tends to become uneven, and it becomes extremely difficult to achieve an accurate piece wise variation of delay time between the taps. Nowadays, delay lines of the above described type whose provided delay time can be varied in steps of 50 ps are being commercially marketed, but this is nearing the practical limit for such a design, and furthermore the range of delay time available is restricted. In fact, although the demand, potential as well as actual, is extremely strong, it has proved to be extremely difficult to apply this design for an electromagnetic variable delay line to the provision of an 8-bit delay line, such as for example one the delay time provided by which can be varied from zero to 12,750 ps in steps of 50 ps. Further, a demand has evolved for an electromagnetic variable delay line the delay time provided by which can be varied in steps of less than 50 ps, for example in steps of 10 ps, but no such products are currently available due to the above mentioned problems.
FIG. 9 shows an alternate example of conventional electromagnetic variable delay line. In this prior art delay line, which is denoted as DLb and the delay time of which can be varied electronically in a continuous fashion, there are comprised an inductance device comprising a solenoid, and a plurality of taps; again, these arrangements are only schematically shown in the figure. Each of this plurality of taps is connected via a corresponding one of a plurality of variable capacitance diodes which are denoted generically as Dv to one terminal of a bypass capacitor Cp, the other terminal of which is grounded. Further, the junction point of all of the variable capacitance diodes Dv and of said bypass capacitor Cp is provided with a control signal (an inverse voltage) -Vd by way of a variable resistance VR.
According to the operation of this electromagnetic variable delay line, by varying the control voltage -Vd which is applied to the variable capacitance diodes Dv by the adjustment of the variable resistor VR, the capacitance of each of said variable capacitance diodes Dv can be adjusted, and accordingly the delay time provided by the delay line DLb can be continuously varied; in other words, the delay time provided by the delay line DLb can be varied by steps which can be in principle be made arbitrarily short.
However, this second type of prior art construction for an electromagnetic variable delay line is also subject to a number of problems.
Particularly, the relationship between the control voltage -Vd which is applied to the variable capacitance diodes Dv by the adjustment of the variable resistor VR and the resulting delay time provided by the delay line DLb is typically non linear, and may in fact be as shown in the graph of FIG. 10, which also relates to the prior art, and is a graph exemplarily showing said applied control voltage Vd along the horizontal axis and the resulting delay time td provided by delay line DLb along the vertical axis. In other words, even if the control voltage -Vd is increased steadily and continuously, the variation of delay time provided by the delay line DLb is not steady, but rather the rate of change of said provided delay time relative to said control voltage -Vd is greater in the later range of high control voltages than in the initial range of relatively low control voltages. Such non linearity is highly undesirable for practical purposes, because it mean that, in order to provide some particular delay time, it is necessary to prepare in advance a table representing the performance data of the delay line DLb as exemplarily shown in FIG. 10, and to perform table lookup in order to determine the appropriate control voltage (such as exemplarily shown as -Vd1) which should be applied in order to provide any required delay time (such as exemplarily shown as td1). This procedure is highly troublesome, time consuming, and complex, and is impracticable to perform in actual applications. Accordingly, this second type of prior art construction for an electromagnetic variable delay line is in practice only suitable for applications in which the accurate delay time is not required to be known in advance, for instance in an application in which the control voltage -Vd is varied until an agreement in the rising slopes of two signals is obtained in adjusting the timing between two signals. However, this second type of prior art construction for an electromagnetic variable delay line is not suitable for an application in which the accurate value of the actual delay time is required to be known.